In recent years, manufacturers have relied upon several processing techniques to manufacture sputter targets from pure cobalt and pure nickel. Manufacturers have traditionally relied upon a combination of hot working and cold working to lower sputter targets' permeability and increase its magnetic pass through flux (PTF). Unfortunately, these processes have limited success with respect to controlling the high-purity target's final magnetic properties. The target's high magnetic permeability and low PTF in turn limit the target's useful thickness to a relatively thin cross section. Furthermore, because the performance of a ferromagnetic sputter target is extremely sensitive to minor variations in magnetic properties, production of a critically-uniform ferromagnetic target is also challenging. Finally, the magnetic properties of a ferromagnetic sputter target are themselves a means to an end—the ultimate measure of improvement is the performance of the target in a sputtering system.
Kano et al., in EP 799905, recognized that strain can manipulate a high-purity cobalt target's permeability. This patent publication discloses a process that relies upon either cold or warm rolling to reduce the target's initial permeability parallel to the target's surface to about 7. Unfortunately, this process also increases the permeability perpendicular to the target's surface.
Snowman et al., in U.S. Pat. No. 6,176,944, disclose another process for reducing permeability of high-purity cobalt targets. This process relies upon: i) controlled cooling to produce an hcp structure; ii) hot working; iii) further controlled cooling to reproduce the hap structure; and iv) cold working to lower the target's permeability. This process lowers the target's initial permeability to less than 9. The cobalt targets produced by this process, however, do not suffer from the severe anisotropic magnetic permeability of the Kano et al. process.
Lo et al., in U.S. Pat. No. 5,766,380, entitled “Method for Fabricating Randomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains and Fine Precipitates” disclose a cryogenic method for fabricating aluminum alloy sputter targets. This method uses cryogenic processing with a final annealing step to recrystallize the grains into a desired texture. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621, uses cryogenic working and annealing to manipulate and enhance crystallographic texture of titanium sputter targets.
Sawada et al., in Japan Pat. Pub. No. 3-115,562, disclose a cryogenic process for lowering the permeability of cobalt alloy targets. These cobalt alloy targets contained a combination of fcc and hcp phases. This process used cryogenic working at a temperature of −196° C. to further reduce magnetic permeability of the two phase cobalt alloy target.
Researchers have explored using cryogenic working to increase the forming limits of aluminum alloy sheet panels. For example, Selines et al. disclose a cryogenic process for deforming aluminum sheet in U.S. Pat. No. 4,159,217. This cryogenic process increases elongation and formability at −196° C. In addition, metal sheet forming industries have exploited high strain-hardening rates to extend the forming limits of sheet metal and improve sheet metal strain accommodation uniformity.